1. Field of Invention
The present invention concerns a capacitor charging power supply. More particularly, the present invention concerns a method for automatically controlling the output of a high voltage, parallel resonant mode power supply operating at a variable, non-audible frequency at or above its resonant frequency, including "charge" modes of operation and a "hold" mode of operation, and having an exceptionally smooth modal (i.e., charge mode to hold mode) transition, very low output ripple, and inherently stable output control characteristics in all operating modes. The present invention also concerns a device for implementing this method.
2. Description of the Prior Art
Known resonant power supplies have been used in various topologies for supplying power to various types of loads. A parallel resonant power converter typically has a transistor switching network connected to a resonant tank circuit having an inductor and a capacitor. The transistor switching network chops a DC input voltage thereby providing an alternating (i.e., a quasi-square wave) voltage across the resonant tank circuit and exciting a resonant current. A transformer is typically used to transfer power from the resonant tank circuit to a full bridge rectifier circuit. The rectified voltage is then applied to the load.
With such known resonant power supplies, the output voltage can be regulated by varying the switching frequency of the switching network. The output voltage is maximum when the switching frequency of the switching network is at the resonant frequency of the resonant tank circuit (which is ##EQU1## where L.sub.RT is the inductance of the resonant tank circuit and C.sub.RT is the capacitance of the resonant tank circuit) and decreases as the switching frequency of the switching network deviates from the resonant frequency point.
Capacitive loads are often used in high voltage applications such as laser pumps, spark gaps, igniter sources and other high voltage pulsed load applications. Capacitor charging power supplies must be "short circuit proof" because an uncharged capacitive load presented to the power supply will initially appear as a short circuit, i.e., it will draw a large current. Thus, in the absence of any control response, the output current must remain at some nominal value when the output of the supply is, or appears to be, shorted.
The capacitor charging power supply must also be able to deliver a large amount of energy to the capacitive load to rapidly charge it. Once the capacitive load is adequately charged, the supply must also be able to maintain the charge of the capacitor with a minimum amount of output voltage ripple. Otherwise, excessive output voltage ripple may cause untimely and inappropriate pulsing of the capacitive load due to fluctuations in the voltage across it. For example, if the voltage varies from pulse to pulse, the energy delivered to the load capacitor will vary. This variation is undesirable because it may affect the consistency of a process controlled by the load capacitor. For example, a capacitor may be charged, and then discharged into a laser having an output used to expose semiconductor wafers to create an image of the integrated circuit pattern. Lasers are highly non-linear devices and a small variation in their input voltage may cause a large variation in their output power. Thus, output voltage ripple by the power supply may cause variations in the output power of the laser which in turn may cause undesirable exposure changes. The variation of the output caused by output ripple may also disadvantageously vary the time at which an energy pulse is delivered by the load capacitor. For example, some pulse systems include saturating magnetic components which operate at a fixed V*T (voltage*time) product. Thus, variations of an input voltage of these components may cause time variations in their outputs. This variation, known as "time jitter," may cause time synchronization problems for example. Lastly, the power supply must be able to smoothly transition from rapidly charging the capacitor to maintaining the charge of the capacitor.
Known capacitor charging power supplies have used two modes of operation, though they suffer from some or all of the problems addressed by the present invention. For example, the power supply discussed in U.S. Pat. No. 5,121,314 (hereinafter "the '314 patent") uses two operating modes in a fixed-frequency, series-resonant, high voltage capacitor charging power supply. The power supply of the '314 patent operates in a first "full charge mode" at a 100% duty cycle with an open control loop to apply maximum power to the capacitive load until the desired voltage is reached, whereupon the duty cycle is reduced abruptly to 13% or less. This abrupt change in the duty cycle precludes a smooth modal transition. Moreover, the abrupt change in the output voltage (approximating a discontinuity) caused by the abrupt change in duty cycle creates a high frequency component which requires a more accurate feedback control. Finally, the device discussed in the '314 patent operates at a fixed switching frequency. Thus, the resolution of the output control is limited by the resolution at which the duty cycle can be changed.
In view of the aforementioned problems with known power supplies for supplying capacitive loads, a short circuit proof power supply is needed. The power supply should also be capable of charging a capacitive load at a rapid rate (e.g., since a capacitive load may be discharged at a rate of several KHz, the power supply of the present invention should be able to recharge the capacitive load in a time on the order of a msec, and preferably on the order of tens or hundreds of .mu.sec) to a desired voltage. At the same time, the power supply should avoid undesirable, and potentially load-damaging, overvoltage or overshoot conditions. The power supply should be able to precisely maintain the capacitive load at the desired voltage level with extremely low output ripple (preferably 0.05% or less). The power supply should operate at frequencies above the audible range. The power supply should also responsively adjust to AC or DC input voltage fluctuations such that these fluctuations are not transferred to the load.
An object of the present invention is to provide an apparatus, having the above-mentioned desirable properties, for converting a first input DC voltage (300 V DC, for example) to a second output DC voltage (40 KV DC, for example) for charging a load capacitor.